(a) Technical Field
The present disclosure relates, generally, to a fuel cell system applied to a vehicle. More particularly, it relates to a method for controlling the operation of a fuel cell system, which can prevent flooding in a cathode of a fuel cell stack during the operation of the fuel cell stack at low temperature.
(b) Background Art
One of the most attractive fuel cells for a vehicle is a polymer electrolyte membrane fuel cell (PEMFC), which comprises a membrane electrode assembly (MEA), a gas diffusion layer (GDL), a gasket, a sealing member, and a bipolar plate. The MEA preferably includes a polymer electrolyte membrane through which hydrogen ions are suitably transported. An electrode/catalyst layer, in which an electrochemical reaction takes place, is suitably disposed on each of both sides of the polymer electrolyte membrane. Preferably, the GDL functions to uniformly diffuse reactant gases and transmit generated electricity. Preferably, the gasket functions to provide an appropriate airtightness to reactant gases and coolant. Preferably, the sealing member functions to provide an appropriate bonding pressure. Preferably, the bipolar plate functions to support the MEA and GDL, collect and transmit generated electricity, transmit reactant gases, transmit and remove reaction products, and transmit coolant to remove reaction heat, etc.
The fuel cell stack preferably consists of a plurality of unit cells, each unit cells including an anode, a cathode and an electrolyte (electrolyte membrane). Hydrogen is supplied to the anode (“fuel electrode”, “hydrogen electrode” or “oxidation electrode”) and oxygen-containing air is supplied to the cathode (“air electrode”, “oxygen electrode” or “reduction electrode”).
Preferably, the hydrogen supplied to the anode is dissociated into hydrogen ions (protons, H+) and electrons (e−) by a catalyst disposed in the electrode/catalyst layer. The hydrogen ions are transmitted to the cathode through the electrolyte membrane, which is a cation exchange membrane, and the electrons are transmitted to the cathode through the GDL and the bipolar plate.
At the cathode, the hydrogen ions supplied through the (polymer) electrolyte membrane and the electrons transmitted through the bipolar plate react with the oxygen-containing air supplied to the cathode to produce water. Accordingly, migration of the hydrogen ions causes electrons to flow through an external conducting wire, which generates electricity and heat.
In the PEMFC system applied to a vehicle, the temperature of the fuel cell stack is low before the temperature of the fuel cell system reaches an optimal temperature (normal temperature above a predetermined temperature) after start-up, and thus the amount of water condensed in a cathode channel is increased, which can cause various problems such as flooding due to the water condensation.
Further, when the amount of condensed water or product water is suitably increased in the fuel cell stack, this water remains in the fuel cell stack together with water condensed after shutdown of the fuel cell system, and as a result, the condensed water freezes at a temperature below the freezing point during the winter season, which makes it impossible to perform a cold start.
Accordingly, it is necessary to provide a method to suitably minimize the production of condensed water in the cathode channel, thereby preventing flooding in the fuel cell stack, and improving the operation stability and durability of the fuel cell stack, and ensuring the startability even when the vehicle is shut down at a temperature below the freezing point or cold started thereafter.
Conventionally, a method of increasing the temperature of the fuel cell stack by consuming the current of the fuel cell stack using loads connected in parallel to the fuel cell stack is employed.
However, although this method has the advantage that the temperature of the fuel cell stack can be suitably increased within a short period of time, the energy generated from the fuel cell stack is all consumed as heat energy, which suitably reduces the energy efficiency. Moreover, this method has its limitations in preventing water condensation and flooding in the cathode channel.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.